invention relates to a fiber-optic gyroscope of the type having a light source coupled through a directional coupler to a fiber optic coil. Light from the light source is split into two beams which circulate in opposite directions through the fiber optic coil, and the phase of the output signals is analyzed to determine the rate of rotation of the gyroscope.
A fiber-optic gyroscope of this type is disclosed in U.S. Pat. No. 4,440,498, in which the directional coupler has six gates arranged in groups of three on both sides of a coupling area. The coupling area in this case may be generated by the pressing or melting of optical fibers disposed next to one another. A light source and two receiver diodes arranged symmetrically on either side of the light source are connected on one side of the directional coupler. On the other side, a fiber coil consisting of a long optical fiber is connected to the outer gates, the center gate, which is opposite the connection for the light source, remaining free and being connectable, for example, with a monitoring circuit in which fluctuations of the intensity of the light source are determined and are taken into account in the measuring results of the fiber-optic gyroscope.
This known fiber-optic gyroscope works as a result of the special development of the directional coupler close to the so-called quadrature point, in which the output signals of the receiver diodes have a sinusoidal dependence on the Sagnac phase so that this fiber-optic gyroscope has maximum sensitivity at low rates of rotation.
Conventional fiber-optic gyroscopes which work with 2.times.2 couplers, i.e., directional couplers with two gates respectively on each side of the coupling area, furnish output signals which depend on the cosine of the Sagnac phase. No differentiation can therefore be made between a positive and a negative rotating direction of the gyroscope. In order to obtain a sensitivity to low rates of rotation in this case, a non-reciprocal phase shifting by pi/2 must be generated at one end of the fiber coil, for example, by phase modulation, so that the cosine dependence of the interference signal becomes a sine dependence. The disadvantage of this type arrangement is the required additional modulator which operates, for example, piezomechanically or electro-optically.
It is an object of the invention to provide a fiber-optic gyroscope of the above-mentioned type which automatically operates in proximity of the quadrature point.
According to the invention, this object is achieved by using an eight-gate directional coupler having four arms respectively leading to gates on both sides of the coupling area, the power distribution to the four output gates taking place with a specific relative phase shifting between the output signals of the coupler. As a result of such defined phase relations between the output amplitudes of the directional coupler, the operating point of the fiber-optic gyroscope may be placed in the quadrature point.
The directional coupler is preferably made of four monomode fibers which rest against one another in the coupling area and are melted on by uniform heating. In this case, the resulting coupling area is preferably drawn as a biconical taper. Before the melting-on, the monomode fibers may also be twisted together in the coupling area. As a result of these measures, particularly by the tapering of the four fiber cores in the coupling area generated by the biconical drawing, the light guided in the fiber cores by total reflection is guided with less power. By means of such an evanescent coupling of the amplitudes lapping into the cladding area of the optical fibers to the adjacent fibers cores, the desired distribution of energy takes place between the four optical fibers. The characteristic of the 4.times.4 coupler which is essential for the desired method of operation of the fiber-optical gyroscope in or close to the quadrature point, in addition to the power distribution to the four outputs, is a specific relative phase shifting between the output signals of the 4.times.4 coupler.
The advantage of such a 4.times.4 coupler in comparison to a known 3.times.3 coupler is the fact that the signal of the additional receiver can also be used directly for the detection of the rate of rotation which, in the case of a 3.times.3 coupler, would be possible only by means of an additional 2 .times.2 coupler which would branch off the light reflected into the light source to an additional receiver.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.